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The high resolution three-dimensional x-ray structure of the metal sites of bovine heart cytochrome c oxidase is reported. Cytochrome c oxidase is the largest membrane protein yet crystallized and analyzed at atomic resolution. Electron density distribution of the oxidized bovine cytochrome c oxidase at 2.8 Angstroms resolution indicates a dinuclear copper center with an unexpected structure similar to a [2Fe-2S]-type iron-sulfur center. Previously predicted zinc and magnesium sites have been located, the former bound by a nuclear encoded subunit on the matrix side of the membrane, and the latter situated between heme a sub 3 and Cu sub A , at the interface of subunits I and II. The O sub 2 binding site contains heme a sub 3 iron and copper atoms (Cu sub B ) with an interatomic distance of 4.5 Angstroms; there is no detectable bridging ligand between iron and copper atoms in spite of a strong antiferromagnetic coupling between them. A hydrogen bond is present between a hydroxyl group of the hydroxyfarnesylethyl side chain of heme a sub 3 and an OH of a tyrosine. The tyrosine phenol plane is immediately adjacent and perpendicular to an imidazole group bonded to Cu sub B , suggesting a possible role in intramolecular electron transfer or conformational control, the latter of which could induce the redox-coupled proton pumping. A phenyl group located halfway between a pyrrole plane of the heme a sub 3 and an imidazole plane liganded to the other heme (heme a) could also influence electron transfer or conformational control.
Bovine heart cytochrome c oxidase is a large multicomponent membrane protein complex with molecular size of 200 kilodaltons comprising 13 different polypeptide subunits. Located in the subunits are two heme A moieties, two redox active copper sites, one zinc, one magnesium, and possibly some phospholipids as the intrinsic constituents (1, 2). This enzyme is one of the most intriguing biological macromolecules in the cell. As the terminal enzyme of biological oxidation, it reduces O sub 2 to H sub 2 O at an active site with the four redox active transition metals coupling to a proton pumping process across the mitochondrial inner membrane (2, 3). The bright green color of the enzyme, which is due to the heme iron and which changes...